1. Field of the Invention
The present invention relates generally to a voice-switched speakerphone coupled to a public switched telephone network, and more specifically to such a speakerphone having an auxiliary attenuator arrangement.
2. Description of the Prior Art
A speakerphone offers the advantage of hands-free telephoning by replacing the familiar handset with a separate microphone and loudspeaker. The convenience of such an arrangement justifies extensive efforts to overcome its acoustic and transmission limitations. In order to overcome the disadvantages inherent in the speakerphone, voice switching has been proposed.
Before turning to the present invention it is deemed advantageous to describe in brief a conventional voice-switched speakerphone used in a mobile telephone network with reference to FIGS. 1 and 2. FIG. 2 will again be referred to later when discussing the instant invention.
In FIG. 1, a voice switching circuit 8 is coupled to a loudspeaker 10 and a microphone 12. The loudspeaker 10 is coupled to a receiving path 14 with a variable gain amplifier 16 forming part of the circuit 8, while the microphone 12 is coupled to a transmitting path 18 with another variable gain amplifier 20 also forming part of the circuit 8. The amplifiers 16, 20 are respectively connected to a receiver 22 and a transmitter 24 both of which are coupled via a duplexer 26 to an antenna 28 and then to a public switched telephone network (not shown).
The voice switching circuit 8 further includes a hysteresis comparator 26 and two signal level detectors 28, 30 each of which has a controllable gain. Each gain of the detectors 28, 30 is previously set to a suitable value considering an overall system in which the speakerphone is installed.
The signal level detectors 28, 30 are arranged to receive the loudspeaker and microphone voltages, respectively, and applies the outputs thereof to the hysteresis comparator 26.
Generally, the voice switching allows substantially one direction of transmission to be fully active at a time. It is assumed that the hysteresis comparator 26 is initially supplied with the outputs (Vdr, Vdt) of the detectors 28, 30. If the output Vdr is larger than Vdt, the comparator 26 issues a high level control signal which sets the amplifier 16 to a full gain and the amplifier 20 to the minimum gain, which is referred to as a receive mode. With the receive condition, even if the output Vdr falls and crosses Vdt, the receive mode is maintained due to the hysteresis characteristics of the comparator 26. That is, the comparator 26 continues to issue the high level control signal. Thereafter, if the output Vdr eventually falls below a critical level of the hysteresis curve, the output of the comparator 26 is switched over to a low level control signal by which the amplifier 16 is set to the minimum gain thereof and the amplifier 20 to a full gain. Thus, the speakerphone, which includes the loudspeaker 10, the microphone 12 and the circuit 8 in the instant case, is brought into a transmit mode. The above-mentioned operation which depends on the previous history induced by the comparator 26 is also applied to the other case where the speakerphone 8 is transferred from the transmit mode to the receive mode.
FIG. 2 is a known switching diagram for analyzing the performance of a speakerphone. This graphical method is helpful and offers the advantage over an algebraic analysis of showing at a glance the condition of the speakerphone.
In FIG. 2, the abscissa represents the voltage at the terminal of the loudspeaker 10 and the ordinate the voltage across the microphone 12. The plane is divided into three regions: transmit, hysteresis and receive regions (T, H and R) by two transition lines A, B. Solid line C is plotted to indicate a line on which the microphone and loudspeaker voltages are identical with each other. The mode which the speakerphone assumes in the hysteresis region H, is determined depending on the previous history. That is, if the receive mode is the previous one then the speakerphone 8 remains same in the hysteresis region H, and vice versa.
Returning to FIG. 1, a broken line 40 indicates an acoustic coupling. Incoming speech signal received at the loudspeaker 10 is fed back through the acoustic coupling 40 to the microphone 12. On the other hand, a broken line 42 reflects network echoes which include feedbacks from hybrid circuits provided in exchanges (for example), a distant talker echo, etc.
The gains of the detectors 28, 30 should be appropriately adjusted for the naturalness of the conversation in the voice-switched communication. The transition lines A, B shown in FIG. 2, are plotted according to the voice-switched operations defined by the gains of the detectors 28, 30. A point P1 indicates a network echo level detected as a voltage Vr1 at the terminal of the loudspeaker 10 when a microphone voltage Vt1 is inputted, while a point P2 represents an amount of acoustic coupling detected as a voltage Vt2 at the terminal of the microphone 12 when a loudspeaker voltage Vr2 is applied to the loudspeaker terminal. In order to assure the correct voice switching operations of the circuit 8, the transition lines A and B have to be determined in a manner that the point P1 is located in the transmit region T and the point P2 in the receive region R.
It is usual to set the gains of the detectors 28, 30 assuming that a distant talker uses a handset and holds the microphone thereof close to his or her mouth and places the speaker against his or her ear. Further, it is a current tendency to narrow the width of the hysteresis region H (the amount of loss during switching) to improve the naturalness of the conversation. On the other hand, in the case where a distant talker also uses a speakerphone, the loudspeaker voltage Vr1 due to the network echo increases up to a loudspeaker voltage Vr3. In other words, the point P1 moves to a point P3 which is located in the receive region R. Thus, the voice switching circuit 8 fails to implement the correct operations thereof and thus invites blocking. Furthermore, "singing" or "howling" also occurs due to the narrow width of the hysteresis region H.